1. Field of the Invention
The present invention relates to a vacuum casting apparatus in which a molding cavity is reduced in pressure substantially to a vacuum and upon opening a gate, a molten metal is charged into the molding cavity at a high speed due to the resultant pressure difference.
2. Description of Related Art
As one example of a casting apparatus capable of casting products of a high quality and a low cost, a vacuum casting apparatus (named by the present applicant as a Vacuum Precharged Closed squeezed casting apparatus) was proposed by the present applicant in Japanese Patent Publication HEI 2-155557.
The proposed casting apparatus includes a molding cavity which can be reduced in pressure substantially to a vacuum, a molten metal supply passage, a gate piston for opening and shutting a passage communicating the molding cavity with the molten metal supply passage, and a pressure pin assembled in the gate piston. In the proposed casting apparatus, the molding cavity is reduced in pressure to a vacuum. Then, the gate piston is opened to charge a molten metal from the molten metal supply passage into the molding cavity at a high speed. The gate piston is then closed to shut the molding cavity filled with the molten metal, and the pressure pin is operated to pressurize the molten metal in the molding cavity before the molten metal solidifies. In this apparatus, since the molding cavity is at a vacuum, the mixing of air in the molten metal is prevented, so that casting defects due to bubbles in the metal are avoided. Further, since the charging speed is high, running of the molten metal in the molding cavity is improved and the production of slimmer cast products is made possible.
However, the following problems yet remain with the above-described vacuum casting apparatus:
First, since the mechanism for shutting the molding cavity from the molten metal supply passage and the mechanism for pressurizing the molten metal in the molding cavity are separate from each other, provision of both mechanisms is accompanied by a restriction in space for providing both mechanisms, an increase in cost, and complication of structure.
Second, since the gate piston shutting process is not followed by the pressurizing process in a continuous fashion, it is difficult to properly determine the timing for initiating the pressurizing step appropriately. For example, if the pressurizing is initiated too early, the pressure pin would operate before the molding cavity was shut so that pressurization would be impossible. On the other hand, if the pressurization is initiated too late, the molten metal would begin to solidify before the pressurization, so that molding defects would be generated.